A personal viewpoint by A.Y. Chmoulian, associate at Royal Haskoning. This paper was first published in GE’s July 2007 issue.
Retaining wall design theory is a complicated subject with a long history. The problem was, for a long time, solved exclusively in terms of Coulomb’s approach, that is, through equilibrium calculations based on post-failure soil pressures. The geometry of the wall reflecting its equilibrium could then be enhanced to create a desired margin of safety on wall stability. In a similar way, direct factoring of the calculated stresses would ensure the necessary margin of wall structural strength.
The main disadvantage of this method was its oversimplified approach to the soil-structure interaction. However, most of the sophisticated methods of soilstructure interaction analysis were not capable of dealing with the conventionally used definitions of factors of safety on wall stability. Hence, the advances in numerical analysis required modification of the safety factor approach to ensure compatibility of the results.
An easy way forward was available through factoring soil strength and was often adopted. The idea is very simple and clear for wall stability: if the soil strength is reduced by a factor and the wall is still standing - the problem is solved.